The World Wide Web is a global information universe[BL92]. Links are used to represent
all sorts of subtle, rich, and and even ambiguous references.
One one web page, we might find...

I saw a great <a
href="http://www.imdb.com/title/tt0091605/"
>movie starring Sean Connery</a>

... while another says...

The <a
href="http://www.imdb.com/title/tt0091605/"
>IMDB page on "The Name of the Rose"</a>
is a great source of information.

Meanwhile, the Web Architecture document[webarch] says, By design a URI identifies one
resource. Which does http://www.imdb.com/title/tt0091605/ identify, then,
the movie, or the page about the movie? The pragmatic answer is: it
doesn't matter that much, provided the community of people making the
links and serving the information agree (to a reasonable extent) on
a set of terms and their meanings.

Human readers are quite robust when it comes to understanding puns
and ambiguously indirect references, but computers are not; in a C
program, the difference between *p and **p is the
difference between a useful computational result and a crash. Even
for human readers, there are limits. If visiting
http://www.imdb.com/name/nm0000225/ with a web browser showed
Christian Slater's photo and filmography, and a writer used that
address to refer, indirectly, to Sean Connery, readers would likely
feel that Grice's Maxim of
Manner[Gr89], Avoid ambiguity, had been
violated.

The design of the Web of documents we have today is the result of
taking the simplest features of hypertext designs from 15 to 20 years
ago, adding globally scoped Uniform Resource Identifiers (URIs), and
relaxing link consistency constraints. The social dynamics of the Web
include lots of people agreeing to just a few design constraints in
order to get a significant return on their investment, whether from
reading or writing or both.

By analogy, the Semantic Web involves starting with simple database
and logic designs and using URIs for column names and symbol
terms. Which constraints need relaxing and which social norms will
result in exponential growth are still open questions.

The W3C Technical Architecture Group (TAG) is chartered to document
and build consensus around principles of Web architecture. This
paper gives a pragmatic theory of reference in the form of some
principles established by the W3C TAG and some personal
conjectures about issues that are still open.

An analysis of httpRange-14

One of the most intensely debated TAG issues is httpRange-14:
What is the range of the HTTP dereference function?. The dicussion
of the issue is almost all publicly recorded, but following it is
challenging, not only because of the quantity, but because of the
diverse backgrounds of the participants, leading to much
miscommunication.

Perhaps a formal analysis is an effective way to
summarize. For example, the simple logical statement that
http://purl.org/dc/elements/1.1/title is an
rdf:Property was a matter of some dispute.

Meanwhile, Tim Berners-Lee argued that HTTP URIs (without #)
should be understood as referring to documents, not cars. To
formalize this position, we'll use some string manipulation properties
from N3[N3CWM2003] and set aside the
prohibition against literal subjects, going just beyond turtle into
N3. Also, we use the log:uri property, which is similar to
the name quoting function in KIF[KIF].

This position is quite clearly inconsistent with the DCMI schema.
Berners-Lee was unable to persuade a critical mass of the TAG to
accept this position. The utility of the dublin core vocabulary was
apparent and the argument against its use of hashless HTTP URIs
included few practical consequences. Plus, the constraint seems to
encroach on the very important principle of opacity of URIs. Any
general-purpose algorithm for finding out the nature of a resource
starting from only its URI is a constraint on how URIs are
minted.

Further, Mark Baker asserted the right to assign any meaning at all
to URIs that he owns. In particular, he said that
http://markbaker.ca/ denotes his very self. To capture this
position, we will borrow from the FOAF[FOAF] vocabulary:

<http://markbaker.ca/>
a foaf:Person;
foaf:name "Mark Baker".

This is also inconsistent with Berners-Lee's position, since
Berners-Lee held that foaf:Person is disjoint with
tbl:Document.

Meanwhile, the DCMI showed some willingness to cooperate with those
who hold that RDF Properties and web pages are disjoint; they arranged
for HTTP
redirections[RFC2616], rather than 200
OK repsonses, in reply to GET requests to
http://purl.org/dc/elements/1.1/title.

Let's introduce some terms for discussing the HTTP protocol. To
dereference http://site.example/path, it's typical to
make a TCP connection to port 80 of site.example and
send a GET /path request. If the reply is...

It is stipulated by all parties in the httpRange-14 discussion that
in this case, the hello world. body of type
text/plain is a representation of
http://site.example/path. We can state the general rule as:

When asked if tbl:Document included the targets
of http POST messages, Berners-Lee said yes, and agreed, to
some extent that the that the term document is misleading.
The TAG coined the term Information Resource. The
term is not completely defined, but the 15 Jun 2005 decision
of the TAG to address httpRange-14 says:

The TAG provides advice to the community that they may mint http URIs for any resource provided that they follow this simple rule for the sake of removing ambiguity:

If an http resource responds to a GET request with a 2xx response, then the resource identified by that URI is an information resource;

If an http resource responds to a GET request with a 303 (See Other) response, then the resource identified by that URI could be any resource;

If an http resource responds to a GET request with a 4xx (error) response, then the nature of the resource is unknown.

We consider it implicit in this decision that it applies to the
case of a dereferencing an ftp: URI as well, so the following
single triple states the position of the TAG quite concisely with
respect to the terminology developed so far:

w:representation
rdfs:domain w:InformationResource.

Since DCMI does not claim that dc:title is a
w:InformationResource, Berners-Lee is able to endorse
their claim that dc:title is an rdf:Property (for
example, in his tabulator implementation, announced in [BL05]) while maintaining his position that
rdf:Property is disjoint from
w:InformationResource.

Note that the TAG has not taken a position on whether
w:InformationResource intersects with
rdf:Property.They do say, Other things, such as cars and
dogs [...] are resources too. They are not information resources,
however [...] which strongly suggests that
w:InformationResource is disjoint with
foaf:Person. Since Mark Baker's
server responds with ordinary 200 OK replies when asked about
http://markbaker.ca/, we have:

So we are have an inconsisitency between the definition of
w:InformationResource and Mark Baker's claim that http://markbaker.ca/ is a foaf:Person.
While anyone can say anything about anything[RDFC], there are consequences to making disagreeable
claims.

The value of agreement

Consider a formalized movie database, where a request to GET
http://fmdb.example/title/tt0091605/ gives:

And suppose John Doe wants a widely-understood identifier for
Christian Slater so that he can use it in a description of a photo
that Jim took of Christian. Jim writes, in
http://photohost.example/jim/photo20.ttl:

How can Jim reuse the information such as the name of the person
depicted in his photo without implyling that the movie merits 7.7 out
of 10 stars? One approach is to not merge the sources, but rather
treat the fmdb data as a separate graph in the SPARQL dataset.
The photo subject's name can be computed using a more explicit query:

This approach is akin to lifting between contexts[G91]. It is a workable apprach to integrating data
from separate contexts, but it is clearly not as straightforward as
merging. The cost of keeping contexts separate demonstrates that
agreement is valuable. The providers of
fmdb.example can lower Jim's cost to use their information if
they publish film ratings that Jim agrees with.

Delegation, consent and causal chains

Jim could, of course, make up his own URI for the subject of his
photo. But then Jim would have to maintain the information about the
actor's name, the movies he starred in, and their titles, at his own
cost. And if others in Jim's position did likewise, consumers of all
this data would be able to correlate photo subjects only at a
significant cost of dealing with URI
aliases. We conjecture that overall utility for the community is
maximized if we adopt a causal
theory of reference[Kr80]. In particular:

To mint a term in the community, choose a URI of the form
doc#id and publish at doc
some information that motivates others to use the term in
a manner that is consistent with your intended meaning(s).

Use of a URI of the form.
doc#id implies agreement
to information published at doc.

Justification of this conjecture is in progress[Co06], using terms such
as intent and impact from the TAG
discussion of extensibility and versioning in Edinburgh in
September 2005.

Advice: Use hash URIs for properties and classes

If you want to write RDF schemas that are consistent with the TAG's
position on httpRange-14, you have three options:

Use the doc#id pattern as above.

Set up HTTP redirects a la dc:title.

Populate the intersection of w:InformationResource
with rdf:Property.

The third option is like publishing movie reviews that people
disagree with. The second option is more trouble than the first,
unless the vocabulary you're describing is very large. So I advise
the first option.

Fragments as sections vs. people

Some argue that Using # [in this way] makes it impossible to
make assertions about parts of documents (e.g. Person A authored
Section #3).[A04]. Indeed, this
is a concern. Let's consider it formally, using FRBR[FRBR], [IFLA]. Suppose
http://fansite.example/baseball is a little database of great
baseball players, with some statistics on
http://fansite.example/baseball#FredPatek, among others:

These two are inconsistent, since the domain of
frbr:creator is frbr:Work, which is disjoint with
foaf:Person, the domain of baseball:average.

So indeed, if we choose URIs of the form doc#id for
people, it leads to inconsistencies with quite reasonable ontologies
if we also use them as document section identifiers. I advise authors
to choose one or the other for each fragment identifier they publish
and be consistent.

In order for this to work with documents published both in RDF/XML
and XHTML, the XHTML media type specifications may need to be ammended
so that authors can opt out of the section-of-the-document meaning
of fragment identifiers that they publish. For example, the
profile attribute from section 7.4.4.3
Meta data profiles of the HTML 4 specification[HTML4] seems like a reasonable opt-out signal.

Populating the intersection of w:InformationResource
with foaf:Person, the way Mark Baker seems to, seems likely
to conflict with useful and reasonable ontologies. I suggest adopting
w:InformationResource rdfs:subClassOf frbr:Work as a
practical constraint. The foaf:primaryTopic relationship
seems particularly useful for relating web pages to things. Rather
than..

Conclusions and Future Work

While the theory presented here gives little by way of rigorously
justified theorems, we hope it gives a coherent and pragmatic approach
to nagivating many issues of identity and reference in the Web.

We hope the formal analysis of the httpRange-14 discussion and
decision, down to a single RDF triple, makes the issue clear without
distorting the positions that it summarizes. We also hope it
demonstrates the utility of RDF, turtle, and N3 as analytic
tools.

When considering which constraints need relaxing and which social
norms will result in exponential growth of the Semantic Web,
mechanisms for expressing trust seem to be critical. We are exploring
approaches using quoting a la if http://weather.example/ny
contains a formula of the form <ny#weather> nws:temp ?X
then lift that claim, <ny#weather> nws:temp ?X into the
knowledge base as a fact. We are exploring these quoting techniques
along with digital signature and proof exchange as a general-purpose
trust infrastructure. These explorations suggest that the law of the
excluded middle, i.e that every formula is either true or false,
should be relaxed in order to allow indirect
self-reference. Constructive proofs seem more promising than those
that use classical fist order reasoning[Co06b].